1. Field of the Invention
The present invention relates to solenoid valves and more particularly relates to an improved construction for coil-actuators for cartridge type solenoid valves which may be subject to severe environmental conditions.
2. Background
For many years the mobile hydraulics industry has been looking for an economical but durable coil used on their valves even when the valves are exposed to harsh environments. For this reason expensive, external frame (“Shell-over”), non-recyclable “Epoxy based Thermoset” coils have been utilized and are still employed for most severe, environmentally unfriendly, applications. However for economic reasons, where the most severe environmental conditions do not exist, less expensive, recyclable “Internal Frame Thermoplastic or Glass Filled Nylon” (hereinafter referred to as Thermoplastic Type or “plastic”) encapsulated coils have taken over most of the mobile market. A few coil/valve manufacturers have attempted to apply various secondary operations to provide products that would survive severe operating conditions, but these have met with little global acceptance. Still others have adopted external frame coils (“shell over”) with thermoplastic encapsulated windings but also have struggled to make these globally accepted primarily due to environmental performance and/or cost/manufacturing variance issues.
A few major OEM Mobile Equipment manufacturers, and some independent bodies (IEC, DIN . . . i.e. IP ratings) have produced various environmental integrity test specifications over a span of many years in order to “get comfort” in applying these products in the various severe environments. The most severe test for coils for valves of this type is Delta Power Company's “Powered Severe Thermal Shock Immersion Test”. Development progress towards satisfactory coil configurations in Internal Frame Thermoplastic type coil configurations, when confronted with a harsh environment, has generally been unsatisfactory due to the inability to satisfactorily seal the coil. The unsatisfactory results are in comparing the Internal Frame Thermoplastic encapsulated coil type in performance to the “Epoxy based Thermoset” encapsulated coils (expensive/not recyclable “shell-over” Type).
These efforts include or have included 1) Post encapsulation impregnations (Acrylic & “Sealant Glues”) intended to seal or fill the pores of base encapsulate, 2) Various bobbin flange edge designs so as to melt and bond to over mould encapsulates, 3) “bobbin-less” manufacturing techniques so as to eliminate the “seams in the encapsulate” altogether, 4) Techniques of over moulding previously moulded assemblies at extreme pressures with the intent to “create a impregnable shell”, 5) use of special coil encapsulate “rubbers”, 6) Local “potting” as a post encapsulate barrier and 7) finally the use of elastomer seals in critical locations. All of these techniques, up until recently, have produced a balance between a performance level somewhat between the “Epoxy Thermoset” “shell-over” design configuration and typical “Thermoplastic” configurations and all equivalent performance solutions provided for substantially higher cost of manufacture when compared to a typical “Internal Frame Thermoplastic” coil configuration.
Another problem area with coil actuators is maximizing coil efficiency by reducing magnetic losses in the moving air gap region (the space between the moving armature, and adjacent flux projecting surfaces). The critical idea is to increase the magnetic attraction/repulsion force for a given current input to the coil without exceeding the coils heat dissipation limit while reducing the space required for the coil-actuator.
Other encountered problem areas in coil constructions of the Thermoplastic encapsulated coil types is the sealing of the exit of the coil lead wires from the body of the coil-actuator.